Epoxy resin systems that cure at ambient or low temperatures are highly desired by the coatings and adhesives industries. Such systems allow application and repair to occur under a wider range of conditions and extend the working season for many construction and repair operations.
In the past, attempts to enhance low and ambient temperature cures have focused on the use of accelerators and catalysts, such as phenol and its Mannich base derivatives, boron trifluoride and its adducts, strong tertiary amine bases, and strong organic acids. An additional approach has been to incorporate another reactive entity into the formulation, such as a low molecular weight acrylate.
These approaches suffer from a variety of defects in that they utilize reagents that are either highly toxic, corrosive, or not wholly compatible with the rest of the epoxy formulation, thus affecting the final physical properties. A particular problem exists with the use of phenol as an accelerator, since it is highly corrosive to skin and is coming under increasing regulatory pressure.
A. Gross and coworkers have published a series of papers and issued one patent regarding the use of N,N'-dimethylethylenediamine. Their work shows that this diamine gives a partial cure at room temperature, but they emphasize a high temperature post cure is required (DE 3 803 508, Dechema-Monographen, Vol. 119). This group also published a paper (Int. J. Adhesion and Adhesives, Vol. 8, No. 4, Oct. 1988, 225-233) in which they state that secondary amines are less reactive than primary amines. Thus, according to their work, the utility of N,N'-dimethylethylenediamine is that it provides a preliminary, non-crosslinked cure with good workability.
JP 02103221 describes the use of N,N'-dimethylethylenediamine and N,N'-dimethylpropylenediamine in epoxy prepegs.
J. Appl. Polym. Sci., Vol. 23, 1979, 3433-3444 describes the use of poly-N-methyl secondary amines to cure epoxy systems.
U.S. Pat. No. 4,521,583 describes epoxy resin cures effected by using bisurea catalysts derived from poly-N-methyl secondary amines, including N,N'-dimethyl-1,3-propane diamine.
C. Caldo in Chem. Ind. (Milan), Vol. 61, No. 9, 1979, 639-42 describes rate studies of bisphenol A diglycidyl ether with aliphatic secondary amines. In Table 2, rate constants are presented for the reaction of N,N'-dialkyl-1,6-hexanediamine with the glycidyl ether of bisphenol A at 200.degree. C. in tetrahydronaphthalene. These data show that under these conditions, the dimethyl derivative is less reactive than either the diisopropyl derivative or the di-n-butyl derivative and only slightly more reactive than the dicyclohexyl derivative.
In Chem. Ind. (Milan), Vol. 49, No. 10, 1967, 1047-1050, C. Caldo reports that N,N'-dimethylethylenediamine reacts with the glycidyl derivative of piperazine faster than do other N,N'-dialkyldiamines, but no data is reported for ethylenediamine itself. The only primary diamine reported in the study is 1,8-p-diaminomenthane, which is known to have very low reactivity with epoxides because of steric hindrance in the vicinity of the amino groups.